This invention relates to facsimile transmitters of the type utilized in facsimile systems including a transmitter, a receiver and a communications network therebetween. More particularly, this invention relates to facsimile transmitters wherein a document is scanned so as to generate electrical information-bearing signals representing the dark-light variations in the document. These information-bearing signals may then be transmitted over the communications network to a facsimile receiver where the information-bearing signals are converted to marks or images on a copy medium so as to form a copy which is a reasonable facsimile of the original document.
In commercially available, prior art facsimile transmitters manufactured by the assignee of this invention, the amplifier portion of the transmitter is provided with an automatic gain control so as to permit an increase in gain of the amplifier when a diminished amount of light is detected by the photodetecting means of the transmitter. In other words, the automatic gain control of the amplifier means permits a correction in the gain of the amplifier when, due to a malfunction such as reduced illumination of the document or decreased sensitivity of the photodetector, the signal generated by the photodetector appears as a signal representing the detection of a darker region on the document. This is of course desirable in that any malfunction in the transmitter is corrected even though that malfunction may be of a relatively minor nature, e.g., the illuminating means in communication with the document at the transmitter becomes covered with dust or other debris. However, automatic gain control of this type is undesirable in that the detection of an actual dark region on the document can result in the adjustment of gain for the amplifier which will make the actual dark region appear as a white region. This phenomenon may be better understood with reference to copending application Ser. No. 417,797 filed Nov. 21, 1973 now U.S. Pat. No. 3,911,207 which is assigned to the assignee of this invention.
As shown in the aforesaid application, the automatic gain control comprises a feedback path including a fixed resistance in parallel with an electronically controlled resistance comprising an FET (field effect transistor). The FET resistance is controlled or adjusted in response to the charge on the capacitor which is coupled to the output of the amplifier through a transistor. When the transistor is conductive as during detection of a white signal, an increased positive charge on the capacitor renders the FET conductive so as to reduce the feedback resistance thereby minimizing the gain of the amplifier. However, when the transistor is nonconductive, the capacitance slowly discharges and the FET is turned off so as to increase the resistance in the feedback path of the amplifier means thereby increasing the gain. When a dark region of extended width or length is detected, the gain of the amplifier is permitted to rise due to the resistance of the fixed resistor in the feedback path to a level approaching white thereby failing to provide a signal which is capable of creating a facsimile of the dark region of extended length or duration.
In commercially available facsimile transmitters manufactured by the assignee of this invention and comprising fiber optics in the optical path between the document and the photodetector and the document and the source of illumination, filters are utilized to improve the response of the transmitter to dark-light variations on the document. However, the filters in such transmitters have not been effective to compensate the spectral response so that all areas which look dark to the eye appear dark to a photocell since the characteristic of the photocell differs substantially from the eye itself. More particularly, the filters of such transmitters have not been extremely effective at filtering out visible red light which is characteristic red ball-point markings. As a consequence, a relatively dark appearing line or mark on a document which is written in red ball-point pen will reflect a sufficient amount of visible red light so that the photodetector does not detect the presence of a dark region.
Commercially available, prior art facsimile transmitters are also somewhat deficient in their speed of response to a dark-light variation in a document. More particularly, an abrupt change from a dark to a light region may not be accurately reflected in the signals which are generated and transmitted by the facsimile transmitter due to the inability of the circuitry to respond to such an abrupt change.